It has been found in the past that a high percentage of animal waste materials can be converted into valuable animal feed and that such can be accomplished in a closed system without chemical treatment. For example, in my prior U.S. Pat. No. 3,982,499, I have disclosed an animal waste conversion and recovery system which has particular utility in livestock feeding in a confinement facility. As described in my patent, the waste material is flushed from the barn into a sump or basin, from which it is then pumped onto a wedge wire screen filter to separate excess liquid from the solid materials: and additional liquid is separated by means of a rotary press at one end of the screen so as to compress or squeeze excess moisture from the solid material as a preliminary to collection of the remaining solid material in a separate holding area. Any liquids and fine solid particles which pass through the screen are delivered into an aeration tank for aerobic treatment. The aerated liquid and waste material are then recycled through the confinement facility to aid in flushing additional waste material therefrom into the sump.
It is recognized that livestock convert a relatively low percentage of feed consumed into weight gain, the remainder being passed through the animal in the form of excrement. For instance, in the case of beef cattle approximately 28% of the feed consumed by beef cattle on a fattening ration is digested or utilized, and the remainder is passed through in the form of excrement. Accordingly, there is a tremendous economical potential not only in reclaiming the undigested solid portion of the feed, but also to increase the concentration of the total digestible nutrients in the feed.
In animal waste conversion systems now in use, such as, those disclosed in my prior patent, generally the percentage of total digestible nutrient value in the recovered waste has been found to be relatively low and can be largely attributed to the failure to retain many of the fine solids in the waste material, or solids which are not filtered out in the separation stage. A good percentage of highly digestible proteins of high energy source are to be found in the fines or finer solid particles of the waste material and, if separated from the solid waste materials along with the excess moisture or liquid will greatly reduce the total digestible nutrient value in the solid waste material recovered. On the other hand, if some of the valuable and extremely fine solids are filtered through the screen with the excess moisture the fine solids will enhance the aerobic biological digestion process in the aeration tank. In other words, it is extremely important to strike a proper balance between the amount of fines permitted to pass through the screen with the liquids and the amount of oxygen induced into the aeration pit for the biological conversion of the fine solids which do pass through the screen, into single cell proteins. The high percentage of the solids which are removed are for conversion into semi-dry feed supplement. The moisture content of the solids removed is the means of harvesting, and is a part of the single cell protein which is derived from the biological digestion of the fine solids which are taken through the screen passing into the aeration tank.